1. Field of the Invention
The present invention relates to an objective for video disks and, more particularly, to an objective for reading out the signals recorded on high-density information-recording disks (video disks).
2. Background of the Invention
For objectives to be used in reproducing systems for video disks, it is required to warrant resolving power of 1.mu. and the numerical aperture of N.A. 0.35 or over due to the fact that the objective has to read out very small signals recorded with high density. Unlike microscope objectives in general, in case of objectives for video disks, observation by an eyepiece is not carried out but the image formed by the objective is detected by a light-receiving element. Therefore, the quality of the image focused by the objective has serious influence on performance of the reproducing system. Moreover, the objective for video disks has to be compact, light in weight, and low in price and has to be arranged for automatic focusing.
Besides, the light used for the objective for video disks is generally a monochromatic light (for example, .lambda.= 632.8 nm). When transparency for the light of the wavelength to be used is as far as possible high, it is effective for eliminating the noise at the time of amplifying the signals from a detector. Therefore, in order to make transparancy of the light high, it is necessary to provide multi-layer anti-reflection coating on lens surfaces or to make the number of lenses constituting the objective as small as possible. When this problem is considered in connection with the above-mentioned other requirements such as low price and light weight, it is more advantageous when the number of lenses constituting the objective is made as small as possible.